An ultramarathon runner experiences severe muscle fatigue in their quadriceps after 50 miles. Which failure mode explains why continued contraction becomes impossible despite sufficient glycogen stores?

💡 Explanation

Muscle fatigue occurs because intracellular phosphate accumulates during prolonged activity, interfering with the power stroke of myosin cross-bridges. This phosphate accumulation directly inhibits actin-myosin binding. Therefore, cross-bridge failure prevents continued contraction, rather than issues at the NMJ, creatine phosphate effects, or persistent hyperpolarization.

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